A greener process for poly-L-lactic acid production and chemical upcycling under mild conditions using highly active alkali-metal based catalysts

Abstract

Non-toxic potassium and sodium metal compounds have been prepared with a bulky aryloxide ligand MOR 1 y 2 as well, their counterparts containing a crown ether bonded to the alkali metal 3 y 4 and fully characterized. The activity of compounds 1 to 4 as catalysts for the ring-opening polymerization process of LLA have been studied, showing that they are extremely active and able to polymerize LLA within a few minutes under mild conditions, achieving PLLA with high molecular weight, similar to the commercial ones. For derivatives 3 and 4, the crown ether coordination to the alkali metal affects the nuclearity of the compounds and consequently its activity, giving a more controlled polymerization. As well the use of BnOH as co-initiator allowed a better control over the polymerization. Detailed studies of the polymerization mechanism have been performed, which confirmed an anionic mechanism in absence of a co-initiator. Furthermore, the nature of the mechanisms provokes the epimerization of the lactide and the existence of D-isomers in the PLLA. Since the percentage of d-isomer content in the polymeric chain as well as its distribution can strongly change the properties of PLLA a detailed analysis has been performed. As expected, the different isotacticity leads to a strong variation on the thermal properties. We have also compared the mechanical properties of the different synthetized PLLA, and we clearly observed that the epimerization reactions lead to an increase of the PLLA flexibility compared with commercial PLAs. Furthermore, the alkali metal compounds prepared not only polymerize, as well in the presence of an alcohol they can depolymerized PLLA within 15 min to give alkyl lactates, allowing the easy upcycling of commercial PLLAs.